Most of the process analytical methods currently in use require sample withdrawal from a chemical process stream and then delivery of this sample via a sample line to a chemical analysis instrument. Often these sample lines, which are expensive to build and maintain, present problems such as delay or difficulty in accessing the more critical points in the process. At times, the sample is pumped and filtered which further complicates operations.
The need for efficient in-line chemical sensors and probes has long been recognized and has led those skilled in this art to design a variety of such systems The diversity of applications, which range from biomedical and environmental monitoring to industrial process control, requires wide variations in physical structure and also performance specifications of these sensors. Most of the fiber optic based sensors described in the literature are based on an immobilized reagent chemistry at the fiber end-face to provide analyte specificity. These sensors suffer from applicability to limited chemistries and probe-to-probe reproducibility. Moreover, problems related to leaching of the reagent, which require continuous calibration and standardization and fragility of the physical structure, inhibit their use for long term in situ process monitoring and control.